The advent of A4WP (Alliance for Wireless Power) charging systems introduces new coexistence issues with the connectivity & cellular radio technologies implemented in mobile phones today. A PTU 120 (Power Transmitting Unit, typically referred to as a “charging mat”) transfers power wirelessly 121 to a PRU (Power Receiving Unit, typically one or more mobile devices 130 as shown in FIG. 1) using resonant coupling at 6.78 MHz. Along with the fundamental continuous-wave tone used for power transfer, the PTU radiates a wide range of harmonics, some of which fall into RF bands 111 used within the PRU. These harmonics can severely desensitize the RF receivers operating while the mobile device 130 is being charged. User experience would be degraded to a different degree depending on the use case but preliminary testing suggests that using GNSS while the mobile device is placed on a car charging pad would be totally impossible as all four satellite constellations would be impacted by harmonics of the PTU signal. Modern radios (WLAN, BT, GNSS, FMR, Cellular Modem, etc. . . . ) are typically capable of applying specific mitigation technique(s) to combat narrowband spurious responses provided the frequency of the interferer is precisely known. The PTU fundamental frequency (and its harmonics) however are non-coherent with the reference clock(s) within the PRU so the PRU does not know a priori the precise frequency at which the harmonics will fall.
Due to the low fundamental frequency of the PTU, virtually all cellular and connectivity bands will have one or multiple harmonics falling in-band. Additionally, LTE and WIFI radio receivers operating with 20, 40 or 80 MHz channel bandwidths would suffer desensitization due to multiple PTU harmonics falling in-channel. To assess the severity of the problem, harmonics of a PTU to PRU wireless power transfer were measured for reference. Harmonics falling in the FMR band would cause desensitization on the order of 30 dB to the FMR receiver. Harmonic levels measured in the GNSS band (1560-1610 MHz) are in the −90 dBm range which would block any satellite signal acquisition.
It may thus be desirable to provide a technique for an accurate estimation of the fundamental frequency of the PTU.